Simply Supported I beam stability bracing

[NS4U]

Part 2, p 2-13 in the AISC manual talks about stability bracing for ends of simply supported beams sitting on bearing plates.

I am going to be adding stiffeners to the beam to act as lateral bracing, but the code doesn't seem to give any guidance on the thickness/width of the stiffener.

Am I missing something? The load is relatively light so even if I designed the stiffeners to take all of the load they're still going to be pretty small.

 

[msquared48]

If I understand your question correctly, the stiffeners you speak of will help to strengthen the top and bottom flanges against buckling, but will do nothing for the lateral stability of the end connection.

This is usually done by transverse beams or walls at the supports or intermediate along the beams length, to include any floor and roof diaphragm connections to the top flange of the beam that may be present.

Mike McCann
MMC Engineering

 

[GalileoG]

Why is it necessary to stiffen the ends of a simply supported beam if you do not have any end bearing strength concerns? I am not sure what stability has to do with this. I would appreciate it if someone can clarify this matter.

 

[msquared48]

It's called web crippling, in addition to bearing. Bearing has to do with the strength of the flanges and web crippling with the web diaphragm stresses. Two different animals, but related.

Mike McCann
MMC Engineering

 

[NS4U]

No, did you look at that page in the manual?

What is the Lb for beam? The distance between two brace points.

What is the Lb for a beam that has no brace points (ie a beam that sits of bearing plates)?

You need to laterally stiffen the section by providing lateral stiffness at the ends. One such way is a stiffener plate.

Even if the section has sufficient strength for the concentrated load failure modes, like web crippling, stiffeners have to be provided.

My question is I can size the stiffeners for strength, but what are the stiffness requirements?

 

[DWHA]

A rule of thumb that our office uses, is that the stiffener plates are at least the thickness of the web. The thing to remember is that the cost of the plate itself is very minimal, so don't be afraid to use a thick plate.